On ignition devices using an ignition coil, a high discharge voltage is produced or induced in a secondary coil by interrupting a primary current at given ignition timing after having energized the primary current to a primary coil, thus generating an electric discharge between the opposing electrodes of a spark plug with a dielectric breakdown in the air-fuel mixture. In more detail, an excessively high-voltage capacitive discharge is momentarily generated. Subsequently to the capacitive discharge, an induced discharge is generated. During the induced discharge, the secondary current flowing across the electrodes decreases comparatively rapidly into a triangular waveform with the lapse of time from the start of the discharge.
Patent document 1 discloses a technology in which the current value of the secondary current flowing across the electrodes of a spark plug is detected, and it is determined that a misfire occurs when the detected current value of the secondary current becomes a prescribed value or less before expiration of a predetermined time from a generation of an ignition command signal.
However, the Patent document 1 never discloses a correlation between the secondary current and the compression ratio.
On the other hand, Patent document 2 discloses a technology in which cranking operation is performed without fuel injection immediately after a start of an internal combustion engine, and a compression ratio is estimated for each individual cylinder, using a temperature of intake air introduced into each of the cylinders and a as temperature in each of exhaust ports into which exhaust gases are exhausted from the individual cylinders. In the Patent document 2, for instance, a fuel injection amount for each individual cylinder is corrected, using a variation (a dispersion) in compression ratio of each individual cylinder.
However, with the aforementioned prior-art system configuration, a temperature sensor has to be arranged for each individual cylinder. This leads to the more complicated configuration.